docs: def exist_edge(self, graph, orig, dest, idx, branch, turn, tick, extant):
          """Declare whether or not this edge exists."""
          if (branch, turn, tick) in self._btts:
              raise TimeError
          self._btts.add((branch, turn, tick))
          graph, orig, dest = map(self.pack, (graph, orig, dest))
          self._edges2set.append((graph, orig, dest, idx, branch, turn, tick, extant))